Various apparatus are available for amplifying signals. In amplifier applications that involve the amplification and transmission of modulated signals, a premium is placed on amplifier efficiency. In addition, because many applications require a high dynamic range, a premium is placed on the ability to efficiently create a high fidelity reproduction of a signal with a widely varying amplitude.
Communications devices, which often transmit high dynamic range signals, are an example of an application where these qualities are in demand. High dynamic range allows the communications devices to communicate more reliably over a variety of distances, and high efficiency allows the devices to operate longer on a single battery.
One method of achieving increased efficiency is to use envelope elimination and restoration (EER)-type amplifiers. EER is a technique through which highly efficient but nonlinear radio frequency (RF) power amplifiers can be combined with other, highly efficient amplifiers to produce a high efficiency linear amplifier system. The signal to be amplified is split into two paths: an amplitude path, and a phase path. The detected envelope is amplified efficiently in the amplitude path by a class S or other highly efficient power amplifier which operates on the bandwidth of the RF envelope rather than the RF bandwidth. The phase modulated carrier in the phase path is then amplitude modulated by the amplified envelope signal, creating an amplified replica of the input signal.
In EER-type amplifiers the dynamic range is limited in part by the dynamic range of the class S modulator used to amplify the envelope. The class S modulator is a pulsewidth modulated system, so the maximum signal level that can be output is limited by the maximum duty cycle of the pulsewidth modulator. In addition, the dynamic range is limited by the minimum duty cycle. It would be desirable to have a method and apparatus which increases the dynamic range beyond that possible with a class S modulator.
In addition, the efficiency of class S modulators decreases for small signal levels. In class S modulators, a drive power is necessary to operate power devices that create an amplified pulsewidth modulated signal, and when the duty cycle is small, the drive power is a large percentage of the total power consumed. It would be desirable to have a method and apparatus for increasing the efficiency of a pulsewidth modulated system for small signal levels.
Accordingly, a need exists for a power amplifier that efficiently amplifies a RF signal exhibiting a high dynamic range.